1. Field of the Invention
An aspect of the present invention relates to a light sensing circuit, a method of driving the same, and a touch panel including the light sensing circuit, and more particularly, to a light sensing circuit that can determine the brightness of incident external light by sensing current flowing in a photodiode, a method of driving the light sensing circuit, and a touch panel including the light sensing circuit.
2. Description of the Related Art
In general, input devices, such as a mouse or a keyboard, are used to apply an input signal to a computer. Remote controls, which are also input devices, are used to control a device, such as a digital television, in a way that a user can select a specific function. However, users who are not familiar with a mouse, keyboard, and a remote controller have difficulty using these input devices.
Touch panels or touch screens are novel input devices that have been suggested as an attempt to solve the above problem. A touch panel refers to an input device that allows a user to input a command signal by directly touching a display panel using his/her finger or a pen instead of a mouse.
Since command signals can be input by touching a display panel using a finger or the like, users having difficulty utilizing input devices, such as a mouse and a keyboard, can easily use digital devices, such as a computer, using such touch panels or touchscreens. There are a number of types of touch panel technology according to a method of recognizing an external input. Such technologies include capacitive technology, resistive overlay technology, infrared beam technology, surface acoustic wave technology using ultrasonic waves, integral strain gauge technology, piezo electric technology, and light sensing technology. Light sensing technology involves forming a photodiode in a display panel and sensing current generated due to light incident on the photodiode in order to recognize a touch by a finger. The operation of a conventional light sensing circuit will now be explained.
FIG. 1 is a circuit diagram of a conventional light sensing circuit.
Referring to FIG. 1, the conventional light sensing circuit includes a photodiode D, a first reset transistor Tr1, a driving transistor Tr2, a first transistor Tr3, a second transistor Tr4, and a first capacitor C1.
A cathode of the photodiode D is connected to a first node N1. The first node N1 is connected to the driving transistor Tr2, the first capacitor C1, the first reset transistor Tr1, and the first transistor Tr3.
An initialization voltage Vinit is applied to the first node N1 through the first reset transistor Tr1. However, although the initialization voltage Vinit is cyclically applied, since the first node N1 is connected to various elements as described above, the risk that leakage current occurs is high. The potential of the first node N1 does not remain constant due to the leakage current and a deviation occurs in each cycle.
Since the same brightness current can be generated at the same brightness only when voltages applied to the cathode and an anode of the photodiode D are constant, such a deviation makes it difficult to accurately sense brightness.
FIG. 2 is a graph illustrating a deviation in brightness data due to brightness current output from the conventional light sensing circuit of FIG. 1.
Referring to FIG. 2, an output voltage is not constant and a deviation ΔV occurs.
Accordingly, a method is needed for accurately sensing incident light by enabling voltages applied to both the cathode and the anode of the photodiode D to be constant.
Also, sometimes the voltage of the cathode of the photodiode D becomes lower than the voltage of the anode of the photodiode D during initialization, thereby the photodiode D of the conventional light sensing circuit of FIG. 1 is not properly forward biased. Accordingly, there is also a need for a method of preventing such a forward bias problem.